1. Field of the Invention
The present invention relates to a three-dimensional stereolithographic method and apparatus using a photohardenable resin composition. More particularly, the invention relates to a three-dimensional stereolithographic method and apparatus for producing various three-dimensionally shaped products having a size ranging from small size to large size from a photohardenable resin composition with a high shaping precision and a good productivity at a high shaping rate without causing uneven curing.
2. Related Art
In recent years, a stereolithographic method and apparatus which produces a three-dimensionally shaped product by curing a photohardenable resin according to data inputted in a three-dimensional CAD has been put in practical use. This stereolithographic technique has been highlighted because it can easily produce a complicated three-dimensionally shaped product such as model for verifying the external design in the course of design, model for checking the functionality of parts, resin model for producing a mold and base model for producing a mold.
In order to produce a shaped product by a stereolithographic method, a method using a shaping bath has been generally employed. In some detail, a process has been widely employed which comprises preparing a shaping bath filled with a liquid photohardenable resin, and then repetitively effecting a step of selectively irradiating the liquid photohardenable resin with spot ultraviolet laser beam which is controlled by a computer such that a desired pattern is provided on the liquid surface of the shaping bath so that it is photohardened to a predetermined thickness to form a cured resin layer, a step of moving vertically the cured resin layer in the shaping bath so that the photohardenable resin in the shaping bath flows onto the cured resin layer to form a photohardened resin solution layer, and a step of irradiating the photohardenable resin solution layer with spot ultraviolet laser beam to form a cured resin layer until a three-dimensionally shaped product having a predetermined shape and dimension is obtained.
However, the aforementioned related art method involving the use of spot ultraviolet laser beam utilizes a so-called dot drawing process which comprises irradiating the surface of a photohardenable resin with one spot laser beam which is moving along the surface of the photohardenable resin to form a two-dimensional photohardened pattern and thus is disadvantageous in that it takes much time to shape the material, giving a lowered productivity. Further, since the ultraviolet laser device used as a light source is extremely expensive, this type of a three-dimensional stereolithographic apparatus is expensive, too.
For the purpose of eliminating the aforementioned disadvantages of the related art technique, a three-dimensional stereolithographic method using a linear exposure mask having optical shutters arranged in a row therein capable of controlling the screening of light at minute dot area has been proposed which comprises controlling the optical shutters according to a predetermined horizontal sectional shape data while scanning the exposure mask in a direction perpendicular to the orientation direction of the optical shutters to form sequentially one layer of photohardened resin layer (JP-A-4-305438). In accordance with this method, it is not necessarily required that as a light source there be used an expensive ultraviolet laser device. An inexpensive light source such as ordinary ultraviolet lamp may be used. Further, the shaping rate can be raised as compared with the aforementioned related art technique involving the use of spot ultraviolet laser beam. However, in accordance with this method, a step of forming a linear photohardened portion one row by one row in the scanning direction of the exposure mask is repeated by a plurality of times to form one layer of sectional shape pattern. When the scanning speed of the exposure mask is raised, a thoroughly photohardened portion cannot be formed one row by one row, making it necessary that the exposure mask be scanned slowly. Further, this method comprises sequentially forming a photohardened portion one row by one row to form a two-dimensional photohardened layer and thus takes much time to shape the material. Accordingly, this method cannot give a sufficiently high shaping rate and thus leaves something to be desired in productivity.
As another method there is known a method which comprises disposing an image drawing mask formed by a liquid crystal shutter capable of screening and passing light at minute dot area fixed between the light source and the surface of a photohardenable resin composition, and then repetitively effecting a step of forming a mask pattern on the image drawing mask according to one layer of sectional shape pattern to be formed with the image drawing mask suspended, a step of irradiating the surface of the photohardenable resin composition with light through the mask pattern so that the photohardenable resin composition is cured to form one layer of sectional shape pattern, a step of supplying subsequent one layer of the photohardenable resin composition onto the photohardened sectional shape pattern, a step of forming a subsequent mask pattern on the image drawing mask according to one layer of sectional shape pattern to be formed with the image drawing mask suspended, and a step of irradiating the surface of the photohardenable resin composition with light through the mask pattern so that the photohardenable resin composition is cured to form subsequent one layer of sectional shape pattern to form a three-dimensionally shaped product.
In accordance with this method, the irradiation of the surface of the photohardenable resin composition with light and the formation of one layer of photohardened sectional shape pattern are effected two-dimensionally at one time, making it possible to raise the stereolithographic building rate as compared with the aforementioned related art method involving the use of spot ultraviolet laser beam and the method disclosed in JP-A-4-305438 involving the use of a linear exposure mask having optical shutters arranged in a row therein capable of controlling the screening of light at minute dot area.
In order to produce a three-dimensionally shaped product by this method, it is necessary that the distance between adjacent minute dot areas projected on the surface of the photohardenable resin composition from the image drawing mask be not greater than 0.1 mm from the standpoint of shaping precision (resolution). It is therefore necessary that the number of pixels be at least about 2,500×2,500 dots for a shaping area size as small as 250 mm×250 mm or at least about 6,000×6,000 dots for a shaping area size as large as 600 mm×600 mm. However, there are no existing liquid crystal masks (liquid crystal shutters) or digital micro mirror shutters which satisfy the aforementioned resolution requirements. Even such a product, if any, is extremely expensive.
In accordance with this method involving the irradiation with the suspension of the fixed image drawing mask, the fineness of the exposed shape pattern is determined by the fineness (roughness) of the image drawing mask and the enlargement/reduction ratio of the pattern projected onto the surface of the photohardenable resin composition through the image drawing mask. The smaller the enlargement ratio is (the greater the reduction ratio is), the smaller is the distance between light dots on the surface of the photohardenable resin composition and the higher is the fineness of the sectional shape pattern thus formed. On the contrary, the greater the enlargement ratio is, the greater is the distance between light dots on the surface of the photohardenable resin composition and the lower is the fineness of the sectional shape pattern thus formed.
Therefore, the aforementioned method involving the fixing of the image drawing mask can difficultly produce a large-sized three-dimensionally shaped product having an excellent fineness (shaping precision) and thus can be used only to the production of a small-sized three-dimensionally shaped product from the standpoint of fineness (shaping precision).
For the purpose of eliminating the disadvantages of the aforementioned method involving the fixed image drawing mask to allow the production of a large-sized three-dimensionally shaped product using a small-sized liquid crystal shutter, JP-A-8-112863 proposes a method which comprises disposing a liquid crystal shutter (liquid crystal mask) capable of selectively passing or screening light such that they can run in parallel to the liquid level of a photohardenable resin along a plurality of divisions, and then repetitively effecting a step of moving the liquid crystal shutter to a first range in the divided running ranges, a step of irradiating the surface of the photohardenable resin composition with light through the liquid crystal shutter suspended with the light source provided in the rear of the liquid crystal shutter being moved over the range of the liquid crystal shutter to form a cured portion corresponding to the first division, a step of moving the liquid crystal shutter to a second range in the divided running ranges, a step of irradiating the surface of the photohardenable resin composition with light through the liquid crystal shutter suspended with the light source provided in the rear of the liquid crystal shutter being moved over the range of the liquid crystal shutter to form a cured portion corresponding to the second division, and a step of effecting the aforementioned steps until one layer of predetermined sectional shape pattern is formed on the surface of the photohardenable resin composition, until a predetermined three-dimensionally shaped product is formed. In this reference, the linear light source is configured to move on the liquid crystal shutter so that exposures are performed in a divided manner over the photohardenable resin composition.
Further in JP-A-07-290578, it is disclosed that the divided exposures are performed with a linear liquid crystal shutter so as to form a predetermined pattern.
However, the method disclosed in the above cited JP-A-8-112863 involves the repetition of a procedure of movement of the liquid crystal shutter to the first division in the running range, irradiation with the liquid crystal shutter suspended (formation of a photohardened portion on the surface of the photohardenable resin composition), movement of the liquid crystal shutter to the second division in the running range, irradiation with the liquid crystal shutter suspended (formation of a photohardened portion on the surface of the photohardenable resin composition), etc. causing the formation of one layer of cured sectional shape pattern and the repetition of this procedure over a plurality of layers to produce a three-dimensionally shaped product. Thus, irradiation is not effected while the liquid crystal shutter is moving to the various divisions in the running range. Therefore, in accordance with this method, exposure is not effected continuously but intermittently, lowering the shaping rate. Further, this method involves curing of the photohardenable resin composition with the liquid crystal shutter suspended in the various divisions in the running range. Thus, the photohardenable resin composition can be easily cured discontinuously or unevenly at the border of these divisions in the running range. The resulting three-dimensionally shaped product is subject to the occurrence of entire intensity spot, reduction of strength and deterioration of external appearance and dimensional precision.
As for the other references, in JP-A-7-227909, it is disclosed that a planar exposure is performed with a masking device. However, it is not disclosed to move the masking device while exposing the light and to perform exposures in divided manner over the photohardenable resin composition. Japanese Patent No. 2624239 suggests using a cathode-ray tube and a liquid crystal device as the masking device.
An aim of the invention is to provide a three-dimensional stereolithographic method and apparatus which can produce a three-dimensionally shaped product regardless of the size thereof, i.e., even if it is small-sized, middle-sized or large-sized, with a high shaping precision at a high shaping rate and a good productivity while preventing the occurrence of uneven curing.
Another aim of the invention is to provide a three-dimensional stereolithographic method and apparatus which can produce a high quality three-dimensionally shaped product having a high shaping precision and free of uneven curing smoothly at a high shaping rate even if an inexpensive light source such as ordinary ultraviolet lamp is used instead of expensive ultraviolet laser device.